markdown: Improve drafts rendering of custom Markdown features.
markdown: Improve drafts rendering of custom Markdown features.

Improves rendering of drafts to make it look more closer
to normal messages. This is done by passing the draft content
through rendered_markdown so that dynamic elements in the content
get updated before showing drafts modal.

andersk/zulip,rht/zulip,kou/zulip,andersk/zulip,kou/zulip,kou/zulip,zulip/zulip,kou/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,kou/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,zulip/zulip,kou/zulip,kou/zulip,rht/zulip,andersk/zulip,rht/zulip,andersk/zulip,rht/zulip